I watched the Netflix original show Mindhunter recently and it raised some profound correlations among serial killers. Since many of the killers in the show are modeled off real criminals, it serves as a good framework for understanding why someone could possibly become so twisted and evil to the point they kill and mutilate others.
One of the major killers in Mindhunter is Edmund Kemper. He is known as the Co-ed Killer for his choice of victim. Kemper killed 10 people in total, including his mother and grandparents. Kemper's parents separated when he was 9 years old because Edmund's father could not stand living with his mother, and a young Edmund was then raised from then on by his man-hating mother. She belittled him and mentally abused him. Kemper already had a history of violence; abusing and killing animals, but living with an abusive mother turned him into a monster.
Charles Manson is mentioned somewhat occasionally in Mindhunter, but is not the focus of the show. Manson ran a commune/cult which was responsible for the murders of 9 individuals. Like Kemper, Manson was raised by a horrid mother, no father. Manson's mother abused and neglected him. Manson had a very troubled childhood and was regularly humiliated by those who had power over him. That made a monster out of Manson.
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There are more serial killers and murderers in Mindhunter, and all of them have similarities. All of them are male and all of them have parental issues. Absent fathers, abusive mothers, neglectful parents, etc. You can trace all of these problems to poor parenting. Even Jeffrey Dahmer, who grew up with both his parents for a while, had parental issues as his parents went through a divorce. Dahmer's parents know a lot more than what they let on. His string of murders has a root cause in the poor parenting.
The recent mass shooter in Florida, Nikolas Cruz, was adopted by foster parents and the father died shortly afterwards. Male. No father. His mother had communication issues with him. The Florida shooting was entirely preventable because the warning signs were incredibly clear. Not every man without a father will become a killer or a criminal, but the chances are so much higher. Of the 27 most recent mass shooters, 26 of them had no father.
With these types of evil people, it always draws back to their childhood and their parents.